Gas stabilized plasma gun

ABSTRACT

A method and means for more efficiently stabilizing a plasma arc produced within a plasma producing device. Improved arc stabilization is achieved by dissipating the heat generated by the electrodes of the plasma producing device under controlled conditions. This is achieved, in one instance, by direct liquid cooling of the anode and concomitantly therewith indirect liquid cooling of the cathode. The latter is achieved by passage of a liquid coolant around a heat sink positioned in conductive relationship with the cathode. To further insure controlled heat dissipation and thereby maintain a preselected cathode temperature profile, the plasma forming gas, as well as the liquid coolant, are introduced into the plasma producing device in a manner and at a rate such that the tip of the cathode is maintained at a temperature just below the cathode&#39;s melting temperature, the central section of the cathode is maintained at or near the cathode&#39;s oxidation temperature and the rear or base section of the cathode is maintained at a temperature below the cathode&#39;s oxidation temperature.

RELATED APPLICATIONS

This application is a continuation-in-part of patent application Ser.No. 337,005 filed on Mar. 1, 1973 now U.S. Pat. No. 3,851,140 andentitled PLASMA SPRAY GUN AND METHOD FOR APPLYING COATINGS ON ASUBSTRATE.

BACKGROUND OF THE INVENTION

1. Field

This invention is directed to plasma producing devices and particularlyto an improved means and method for producing a stabilized plasma arcwithin a plasma producing device.

2. State of the Art

The use of plasma guns for converting a gaseous medium into a plasmahaving a high temperature and velocity by means of an electrical arc iswell known. Although there are a substantial number of patents relatedto plasma producing devices and particularly plasma spray torches, mostof the torches currently available are extremely sensitive and difficultto control, particularly from the aspect of producing a stabilizedplasma arc. When a non-stablized plasma arc is being produced, theelectrodes become pitted within a very short period of time,necessitiating that the electrodes be replaced on a more frequent basisin order to maintain high operating efficiencies.

OBJECTS OF THE INVENTION

To overcome the above deficiencies, it is a primary object of thisinvention to provide a plasma spray gun and method for dissipating theheat generated by the cathode within a preselected temperature profile.Another object of this invention is to provide a plasma producing devicewhereby the anode is directly cooled and the cathode is indirectlycooled by the passage of a coolant around a heat sink in conductiverelationship with the base of the cathode. A further object of thisinvention is to provide a plasma spray gun whereby the flow of liquidcoolant and plasma producing gas is coordinated during operation toinsure that a preselected cathode temperature profile is maintained.Other objects and advantages of this invention will be more apparentfrom the description which follows.

SUMMARY OF THE INVENTION

The plasma producing device of this invention comprises generally aplasma producing gun having a cathode and an anode section separated bya dielectric section, said sections defining in unison a substantiallyenclosed inner or gas chamber. A bored anode is carried within the anodesection to define a substantially elongated nozzle outlet from theenclosed inner chamber. A cathode, carried by the cathode section,extends into the enclosed inner or gas chamber in a spaced relationshipwith the anode. A liquid coolant passageway passes through the anodesection circumscribing the anode and thence passing to outlet throughconnecting passageways bored in the cathode and dielectric sections. Agas distribution ring is supported within the enclosed inner chamber bythe dielectric section. The gas distribution ring circumscribes thecathode and thereby permits a plasma producing gas to flow about thecathode and into the inner or gas chamber. An electrical energy sourceis provided to produce an electrical arc between the cathode and theanode for converting the gas introduced into the inner or gas chamberinto a high temperature, high velocity plasma. The electrical plasmaproducing arc is stabilized by cooling the anode and maintaining thecathode at a temperature such that a temperature profile substantiallyidentical to that depicted in FIG. 5 is produced. The above cathodetemperature profile is achieved by passing a liquid coolant throughinterconnecting liquid coolant passageways such that the anode isdirectly cooled by the liquid coolant and the cathode is indirectlycooled by passage of the liquid coolant around a heat sink, which is inconductive relationship with the cathode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a three dimensional cut-a-way view of the plasma gun of thisinvention taken along line 1--1 of FIG. 2;

FIG. 2 is a front elevation of the plasma gun shown in FIG. 1 lookingfrom right to left;

FIG. 3 is a back elevation of the plasma gun shown in FIG. 1 lookingfrom left to right;

FIG. 4 is an exploded side cross sectional view of the gun shown in FIG.1;

FIG. 5 is a chart which graphically and pictorially depicts atemperature profile generated by the cathode during operation of theplasma gun shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The plasma gun of this invention comprises in combination three primaryor major sections-- the cathode section 10, the anode section 14 and anintermediate or dielectric section 12. All three sections are describedin detail in the description which follows.

ANODE SECTION

The anode section 14 includes a copper anode nozzle 16 comprising asubstantially cylindrical centrally bored copper piece 18. The inlet end20 of the nozzle is expanded or conically shaped to accommodate inspaced relationship the tip 22 or forward end of a tungsten, orpreferably, a tungsten thoriated cathode 24. The anode has a wide, deepset annular groove 26 along its outer peripheral surface. This annulargroove is in communication with a liquid coolant passageway 28vertically bored in a brass anode holder 30. The anode holder 30 has abored center section 32 which over-rides the inserted copper anode 16,(see FIG. 1). To insure a sealing environment between the nozzle anode16 and anode holder 30, the anode is adapted with a small annular groove34 cut into each end of the anode for receiving an "O" ring 36. Theinlet end 38 of the liquid coolant passageway 28 of the anode holder 30is adapted to receive a liquid coolant line and an electrical line 40which provides the gun with the coolant and electrical power necessaryfor operation of the gun. The front face of the copper nozzle 16 andanode holder 30 is fitted with a brass cover plate 42 having a flaredcentral opening 44 in alignment with the central bored opening 46 of theanode nozzle 16. The cover plate 42 can take on most any configurationand is normally used to hold the anode in position and as an adapter forthe addition of auxiliary equipment, such as a powder feed inlet lineand connector for introducing particulated materials into the exitingplasma gas. Particulated materials such as metals and polymericmaterials can thereby be introduced into the plasma stream for eventualdeposition on a substrate. Other adapting pieces may also be used withthis plasma gun. The brass cover plate is secured to the anode holder bymeans of a threaded opening 47 bored therein and a bolt 48.

As will hereinafter be explained in greater detail, the leading edge 50of the anode, that is, that forward portion leading from the gun's innerchamber, which is also the gun's gas receiving chamber, is rounded orcurved to minimize, if not avoid, a turbulent gas flow. For theembodiment shown in FIGS. 1-4 the leading edge of the nozzle is roundedto about 1/16 to 1/64 of an inch (0.16 to 0.04 cm) and preferably isrounded to about 1/32 of an inch (0.08 cm.).

DIELECTRIC SECTION

Abutting the rear face of the anode section 14 is the dielectric section12 of the plasma spray gun. The dielectric section comprises a gas ringholder 52 having a central or axial bore 54 registering and incommunication with the central bore 46 of the nozzle anode 16. The gasring holder 52 is constructed from a dielectric material such as Nylon,impregnated with titanium oxide or phenolic resins, such as Bakelite.Any material which is nonelectrical or nonconductive and capable ofwithstanding high temperatures may be used. In addition to the centralbore, the gas ring holder 52 contains a radial bore 56 which is incommunication with the central longitudinal or axial bore 54. The radialbore is adpated to receive an inlet plasma producing gas feed line 58which in turn is connected to a gas source (not shown).

A ceramic gas distribution ring 60 is held within the central bore 54 ofthe gas ring holder 52 by a reduced bore 62 located in the forward endof the cathode holder 64. The gas distribution ring distributes theplasma producing gas carried by the inlet line 58 and radial bore 56 ofthe gas ring holder into the gun's inner or gas receiving chamber. Thegas distribution ring 60 is preferably designed with radial and axialbores (not shown) to distribute the gas in a manner such that a majorportion of the plasma producing gas is introduced into the gas receivingchamber as a linear flow, and a minor portion of the gas is introducedas a helical flow component which circumscribes the linear flowcomponent. The gas distribution ring used herein is described in greaterdetail in U.S. Pat. application Ser. No. 337,005 filed on Mar. 1, 1973.

To insure a tight fit between the anode section 14 and the dielectricsection 12, a plurality of small anular grooves 66 are provided alongthe rear section of the anode holder 30 for receiving "O" rings 68.Although "O" rings are depicted as the sealing means between the varioussections of the plasma spray gun, any sealing means capable ofwithstanding high temperatures may be used. Preferably, though, thesealing means will not be of a permanent nature, so as to permitconvenient disassembly of the plasma gun for repairing, cleaning orotherwise modifying the plasma gun as may be desired.

The gas ring holder as earlier indicated contains a central bore 54whereby a tungsten, or preferably a thoriated tungsten cathode 24,having a conically shaped head, is held in spaced relationship with theanode nozzle 16. The gas distribution ring, as earlier noted,circumscribes the cathode 24 in spaced relationship thereto. The gasentering the gas distribution ring is ejected about the cathode throughports in the gas distribution ring and into the inner or gas receivingchamber as was earlier described.

The dielectric section contains an upper, longitudinally bored liquidcoolant passageway 70 aligned with and in communication with the liquidcoolant passageway 28 bored in the anode holder 30. The liquid coolantintroduced into the anode holder is carried within the annular groove 26formed in the anode nozzle 18 and outwardly into the liquid coolantpassageway 28 of the anode holder 30 and then into the liquid coolantpassageway 70 formed in the gas ring holder 52. From there the liquidcoolant passes into the liquid coolant passageway 72 bored in thecathode holder and thence to an outlet line 74 to complete the coolingflow sequence.

CATHODE SECTION

The cathode section 10 comprises a centrally bored brass cathode holder64 having a rear internally threaded end section 76. A threaded copperbase or plug 80 is screwed into the cathode holder, closing off the rearsection of the central bore. The rear face of the base plug 80 containsa slot 81 to facilitate its removal from the cathode holder by means ofa screwdriver. The forward elongated end 86 of the copper base or plugcontains a recessed blind bore 82 for receiving and holding the base 84of the cathode 24. The cathode 24 can be secured to the cathode base orplug by most any means capable of providing a convenient means ofdisassembly. For example, the base of the cathode may be threaded forscrewing into a corresponding threaded opening bored in the end of theplug 80.

The cathode base has a machined down elongated forward section 86 whichis in the path and in communication with the liquid coolant passageways72 bored in the cathode holder. The liquid coolant passes through thedielectric section, enters the cathode holder and passes around theelongated forward section of the cathode base, cooling same. Annulargrooves 88 and "O" rings 90 are also provided in the forward end of theelongated section 86 for contact with a ridge 91 in the cathode holder64 to prevent passage of the liquid coolant into the inner or gasreceiving chamber. The cathode holder 64 is also provided with a radialbore 92 which communicates with the liquid coolant passageway 72. Theradial bore 92 is adapted with an outlet line 74 for carrying the liquidcoolant away from the plasma gun. The outlet line 74 is adapted with anelectrical conduit for carrying an electric current to the cathode inthe same manner the inlet line 40 carries an electrical current to theanode. Additional radial grooves and "O" rings 94 and 96, respectively,are also provided on the forward and rearward face of the cathode holderto maintain a complete seal between the cathode base, the cathode holderand gas ring holder.

All three sections of the plasma gun are enclosed and held in positionby an insulated gun housing 100 secured to a base member 102 by overheadscrews 104. The gun housing is constructed from a non-conductivematerial such as rubber, plastic, synthetic resin and the like. Thevarious sections of the gun are held in positional alignment withrespect to each other by long-stemmed threaded bolts passinglongitudinally through at least two of the gun's main sections.

OPERATION

In operation a radio frequency current is applied to the anode andcathode through their respective electrical connecting conduits carriedby the anode and cathode holder, respectively. The initial high voltageproduces an electrical arc between the cathode and anode. In addition,the electrical arc provides a conductive path which allows for a lowervoltage to be applied to the electrodes and still maintain an arctherebetween. Generally, the electrical arc can be sustained by theapplication of 50-85 volts and 150-400 amperes across the electrodes.Once the arc has been generated and stabilized, the voltage may then befurther reduced to a point where the electrical arc is just beingsustained.

A plasma producing gas is introduced into the inner or gas receivingchamber via the gas distribution ring and the gas inlet line. As the gaspasses through the electrical arc, the gas is ionized, producing what isnormally referred to as a gas plasma. Since the plasma is a highlyenergized material, it is emitted through the nozzle at a temperature ofbetween about 2,000° to 10,000° C., and at a velocity approaching mach1.

As indicated, one of the major problems with plasma guns of the typeherein described, was the difficulty in maintaining a stabilizedelectrical arc. For purpose of this disclosure, the electrical arc canbe said to be stabilized if it is evenly distributed between the tip ofthe cathode and the longitudinal base of the anode nozzle. The arc canbe said to be unstabilized if it moves from one point to the next alongthe longitudinal nozzle bore causing pitting of the nozzle's inner wall.When the arc is unstable, the temperature and velocity of the resultingplasma is likewise difficult to control and to maintain constant.

To maximize stabilization of the electric arc and thereby maintain amore consistant plasma, it has been found that if the cathode is cooledso that it has a temperature profile within the range graphicallydepicted in FIG. 5, the electrical arc and resulting plasma are therebystabilized. Basically it has been found that if the tip of the cathodeis maintained at a temperature of just below the melting point of thematerial from which the cathode is constructed (eg tungsten), a morestabilized, highly efficient plasma can be produced. It has been furtherfound that if the central portion of the cathode is maintained at atemperature at or near the oxidation temperature of the tungsten cathodeand the rear or base end of the cathode is held at a temperature belowthe oxidation temperature of the cathode, improved arc stabilization canbe obtained. When a tungsten cathode is used, the melting point is about3370° C., the oxidation temperature is about 1200° C. and the base ofthe cathode is maintained at a temperature of around 150° C. When thesetemperatures are graphically represented on a semi-log graph, anessentially straight line is generated. The lines on either side of theplotted points represent the range of cathode temperatures which may byused for achieving and maintaining arc stabilization. For example, thetip of the cathode should preferably be held at a temperature of around3000°-3300° C., with the center and base section of the cathodes to beheld between a temperature range of between about 800°-1600° C. and100°-700° C., respectively.

If a thoriated-tungsten, rather than a tungsten cathode is used, a morecomplex temperature profile is generated. With a thoriated-tungstencathode, the thorium, having a lower melting and boiling point thantungsten, (1845° C. and 3000° C., respectively), will achieve maximumion emission at a temperature substantially lower than that of tungsten.This range is depicted in FIG. 5, as being approximately between pointsA and B. Within this range the thorium exhibits its highest ion emissionpotential.

The range between points B and C represent the range wherein maximum ionemission from the tungsten portion of the cathode is achieved. Sincethat section of the cathode between points B and C contain little, ifany thorium, (most of it being boiled off), the thoriated tungstencathode provides a broader base from which ions can be released.

Whether the cathode is constructed from a single pure metal or whetherit is constructed from a mixture of metals, the basis premisehereinbefore set forth is applicable. In other words, the cathode shouldpossess a temperature profile such as that depicted in FIG. 5 for eachmetal present.

To achieve this type of temperature profile, it has been found that themeans used for cooling the anode and cathode, as well as the rate ofplasma producing gas flow, must be coordinated with the voltage andamperage applied to the electrodes. For purposes of this invention it isassumed that the applied voltage and current is held relativelyconstant, leaving the means for cooling the electrodes and the gas flowintroduced into the plasma gun as the controlling variables.

Of the two variables, the means for cooling the cathode and anode havebeen found to be the most critical. The most effective cooling wasachieved by cooling the anode directly and by cooling the cathodeindirectly with the liquid coolant. To achieve the latter, the cathodebase is constructed from a highly heat conductive heat transferringmaterial such as copper. The heat generated by the cathode istransferred to the cathode base through conduction, and the cathode baseis thereafter subsequently liquid cooled with the liquid coolant passingaround the anode and through the anode holder and dielectric sectioninto the cathode section of the plasma gun. With this system the base ofthe cathode is more rapidly cooled than its tip, permitting the tip tobe more easily maintained at a much higher temperature than the base ofthe cathode. With this arrangement the cathode base functions as a heatsink for the cathode.

In addition, a high temperature differential is maintained between theanode and the tip of the cathode. In most cases the anode is held at atemperature of below 500° C. and preferably between about 200°-500° C.It has also been found that the rate of gas flow into the gas chamberalso assists in producing the desired cathode temperature profile, aswell as maintaining the desired temperature differential between thecathode tip and the anode.

This gas flow will vary depending on the type of gas introduced into theplasma producing chamber. If the plasma producing gas is diatomic, e.g.,nitrogen, the gas flow will be between 40 and 150 cubic feet (113.2× 10⁴to 424.5× 10⁴ cm³) per hour, and more preferably between 50 to 80 cubicfeet (141.5× 10⁴ to 226.4× 10⁴ cm³) per hour, after the desired flow ofplasma producing gas is achieved and if the cathode temperature, thatis, the temperature which will provide a temperature profile such asthat shown in FIG. 5, has not been attained, the rate of water coolantflow is increased or decreased, to obtain the desired temperatureprofile.

When the plasma producing gas is nitrogen, the rate of coolant flow willnormally be between about 3 and 5 gallons per minute (13.7 and 22.8liters) assuming that a voltage of between about 50 and 70 volts havinga current flow of between about 150 and 300 amperes is applied to thecathode. When the above operating parameters are applied to the plasmagun hereinbefore described, the cathode will assume the temperatureprofile depicted in FIG. 5.

While the invention has been described with reference to severalspecific embodiments, it should be understood that certain changes inconstruction may be made by one skilled in the art and would not therebydepart from the spirit and scope of this invention which is limited onlyby the claims appended hereto.

I claim:
 1. A method for stabilizing a plasma producing arc produced bya plasma producing device having a spaced apart cathode and anode and ameans for producing a plasma producing arc between said cathode andanode, comprising cooling said cathode in a manner such that a cathodetemperature profile substantially identical to that which is depicted inFIG. 5 is obtained.
 2. A method for stabilizing a plasma producing arcproduced by a plasma producing device having a spaced apart cathode andanode and means for producing a plasma producing arc between saidcathode and anode, comprising cooling said cathode in a manner such thatthe tip of the cathode is just below its melting temperature, the mid orcenter section of the cathode is at or near its oxidation temperatureand the base of the cathode is below its oxidation temperature.
 3. Amethod according to claim 2, wherein plasma-producing gas is introducedbetween said cathode and anode at a selected rate; liquid coolant isintroduced to directly cool said anode and indirectly cool said cathode;and said selected rate of gas introduction and the cooling rates of saidcathode and anode are coordinated such that the temperature of the tipof the cathode is held at a temperature between about 3000° C. and about3300° C., the temperature of the center section of the cathode is heldbetween about 800° C. and about 1600° C., and the temperature of thebase section of the cathode is held between about 100° and about 700° C.4. A method for cooling a cathode in a plasma producing device having agas distribution ring circumscribing said cathode for introducing aplasma producing gas into said plasma producing device whereby a majorportion of said plasma producing gas is introduced as a linear flowcomponent and a minor portion is introduced as a helical flow componentcircumscribing said linear flow component, comprising introducing aliquid coolant into said plasma producing device to cool said cathode ina manner such that the tip of said cathode is just below its meltingtemperature, the mid or center section of said cathode is at or near itsoxidation temperature and the base of the cathode is below its oxidationtemperature.